The use of the electrical resistance technique is widely applied in monitoring material loss occurring in industrial plant equipment and pipelines. This technique operates by measuring the change in electrical resistance of a metallic element immersed in a product media relative to a reference element sealed within the probe body. Since temperature changes affect the resistance of both the exposed and protected element equally, measuring the resistance ratio minimizes the influence of changes in the ambient temperature. If the corrosion occurring in the vessel under study is roughly uniform, a change in resistance is proportional to an increment of corrosion. Although universally applicable, the electrical resistance method is uniquely suited to corrosive environments having either poor or non-continuous electrolytes such as vapors, gases, soils, “wet” hydrocarbons, and non-aqueous liquids.
An electrical resistance monitoring system consists of an instrument usually with data logging functions connected to a probe. The instrument may be permanently installed to provide continuous information, or may be portable to gather periodic data from a number of locations. The probe is equipped with a sensing element having a composition and material processing history similar to that of the process equipment of interest.
Electrochemical noise is a useful, sensitive and non-intrusive technique for corrosion monitoring. Fluctuations of potential or current of a corroding metallic specimen are monitored to gage and understand the corrosion process. Electrochemical noise is used to investigate localized corrosion processes such as pitting or stress corrosion cracking, exfoliation, and erosion-corrosion in either laboratory or diverse and complex industrial environments. During localized corrosion, film formation, passivation breakdown or pit propagation processes generate the electrochemical noise that is observed. The most traditional way to analyze electrochemical noise data has been to transform time records in the frequency domain in order to obtain power spectra with FFT methods.
Inductive resistance probes are similar to ER probes. The weight loss in the sensor element is detected by measuring changes in the inductive resistance of a coil, located inside the element. The inductive measurement technique provides greatly improved sensitivity and earlier detection of corrosion rate changes compared to conventional electrical resistance probes. Inductive resistance probes require temperature compensation, similar to ER probes. Like ER probes, the sensors can be used in a broad range of environments such as low conductivity and non-aqueous environments, where electrochemical techniques are generally unsuitable.
Polarization resistance is particularly useful as a method to rapidly identify corrosion upsets and initiate remedial action, thereby prolonging plant life and minimizing unscheduled downtime. The technique is utilized to maximum effect, when installed as a continuous monitoring system in almost all types of water-based, corrosive environments. The measurement of polarization resistance has very similar requirements to the measurement of full polarization curves.
One drawback all prior art systems and devices have is that the power required to operate them is far too high for sustained, long-term battery operation. The resistances of corroding test coupons are very low, typically on the order of 10 milliohms. Because low average power consumption is a requirement in many applications of corrosion measurement, high-current excitation of the coupons is not an option, while low-current excitation results in signals of microvolt magnitude. Presently available commercial instruments fail to meet the low-power requirement.
Therefore, it would be of great advantage if a system could be invented that would use low-power demand while providing acceptable accuracy.
Another advantage would be if a system could be invented that would avoid offset and thermoelectric potentials at connection points.
Yet another advantage would be a system for measuring corrosion that is more accurate due to elimination of noise and offset from high gain amplification.
Other advantages and features will appear hereinafter.